High-toughness cast-iron for relatively thick castings, and method of producing same



United States Patent HIGH-TOUGHNESS CAST-IRON FOR RELATIVELY THICK CASTING'S, AND METHOD OF PRODUC- ING SAIME Lucien Paras, Billancourt, France, assignor to Regie Nationale ties Usines Renault, Billancourt, Seine, France No Drawing. Ffled Jan. 30, 1959, Ser. No. 700,038

Claims priority, application France Feb. 6, 1953 1 Claim. (Cl. 148-35) In prior patent applications, such as those which have issued as US. Patents Nos. 2,895,860 and 2,899,346, I have described a method of producing ferrous metal castings having novel and improved properties, wherein the graphite is diffused in the form of a large number of small particles, the mechanical properties of the metal thus obtained being such that in many cases the cast-iron of this specific quality maybe substituted for steel. These results are due essentially to the application of a special thermal treatment comprising the following successive steps: austenization, martensitic hardening, germination heating at a critical effect temperature, followed by graphitization heating at a temperature above the temperature of eutectoid transformation.

This method is particularly suitable for the manufacture of thin castings, for example of a thickness of or less.

Now it is the object of this invention to provide specific means whereby the advantageous results obtained with the method broadly set forth hereinabove may be re- 9 Percent Ca 2 to 3.0 Cu 0.1 to 3.0 s 0.01 to 0.20 Si 1 to 2.5 Mo 0.05 to 0.50 P 0.02 to 0.15 Mn 0.1 to 2.5 Mg 0.03 to 0.2 Fe Balance Magnesium acts in this case as an element providing the white structure. This action is paradoxical since it constitutes the essential element of spherical graphitization in the fabrication of the so-called spheroidal castiron. Other whitening elements, other than magnesium, or an association of whitening elements, may be used for achieving the same results, if desired.

In addition, Ca, Te, Se, Sn, Bi, Sb, either separately or in combination, may be used as partial or entire substitutes for magnesium, their quantity being the reactive equivalent of the quantity of substituted magnesium.

In this procedure molybdenum acts as a promoter of martensitic hardening, but other elements characterized by the same property may be substituted therefor in a quantity providing the equivalent capacity of action. In the case considered herein the law of partial or total substitution which was discovered according to the present invention is represented by the equivalence table here- Patented Oct. 24., 1961 2 after, wherein the part or parts by weight of the elements relate to one part by weight of molybdenum:

Silicon acts in this case as a promoter of graphitization, the later occurring during the thermal treatment. Its action may be completed by the addition of one of the following elements:

Titanium, with a maximum quantity of 0.5 (by weight), or

Zirconium, with a maximum quantity of 0.5% (by weight), or

Aluminum, with a maximum quantity of 0.20% (by weight).

(2) The casting is stripped well before it has cooled down completely, and in any case at a temperature above the temperature of eutectoid transformation, for example when the casting temperature ranges from 750 to 1,000 C., the casting is then placed immediately after its stripping in an oven at a temperature ranging from 740 to 850 C.

(3) When the-temperature of the casting has been stabilized in the waiting oven, the casting is hardened by immersion either in oil at a temperature between room temperature and 200 C., or in a salt bath at a temperature differing slightly from the M point, that is, at from to 250 C., the martenstic structure being obtained in this case either by direct hardening or by stepwise hardening, according to the temperature of the hardening medium.

(4) As a possible step, the casting is subjected to a cold treatment at a temperature ranging from 10 C. and C. for the purpose of transforming the austenite likely to remain in the hardened castings into martensite.

(5) The casting is then given a germination treatment at an accurately defined temperature depending on the composition of the cast-iron, this temperature always ranging from 425 to 500 C., for a time period of from 1 to 48 hours. This treatment may be followed by cooling to room temperature.

(6) Finally the casting is subjected to graphitization which is carried out at a relatively high temperature for a sufiicient time period, these two parameters being defined as a function of the composition, casting procedure and thickness of the parts. A reduction of from 850 C. for about 8 hours to 1,100 C. for about 15 minutes may be contemplated, according to circumstances, which is considerably faster than in the case of malleable cast- 1rons.

The specific novelty of the present invention lies in the definitely unforeseen effects due on the one hand to the action of the hardening of the non-cooled casting and on the other hand to the presence of hardening elements.

As a matter of fact, a systematic study carried out according to the lominy test utilizing the end hardening test proved that the influence of the conventional martensitic hardening elements, when the casting temperature is used, the austenization being effected from melting, was increased considerably in relation to that of the same elements obtained by re-heating the cold castings for austenization at a conventional temperature of from 50 to 100 C. above the transformation point when heating the castmg.

The influence of the following elements, which is ligured out in additional martensitic hardened depth per percentage of each element, is shown in the next table setting forth on the one hand the results obtained with the hardening of a non-cooled casting, and on the other hand the results obtained for an austenization by re-heating to 830 C. ofthe cooled casting, followed by a hardening step:

Austenization by reheating H to 830 G. of Austenization of non-cooled cooled pasting,

casting with subsequent hardening .Millimuters With 1% of Mn 16 With 1% of Cu With 1% of Mo M As a consequence of this discovery, high-martensitic hardenability castings are obtainable by using moderate c=2.5s%; Si= 1.34%; Mn=l.08%; Cu=2.0%; =0.35% S=0.009%; P=0.044%; Mg=0.29%; Fe=the balance,

are stripped ten minutes after the'completion of the casting step proper, and cooled for 5 minutes in a calm air atmosphere, before being placed in an oven at 780 C. for 5 minutes for stabilizing their temperature; then these castings are immersed and hardened in oil at 180 C. for 7 minutes and finally cooled in calm air. The hardened casting is white to the heart without any trace of graphite, and its micro-structure consists of martensite with to of austenite, without pearlite to the heart.

The casting is then heated for 4 hours at 450 C. for germination and subsequently heated for 8 hours at 950 C. for graphitization. After cooling, the'casting shows a very homogeneous structure from the heart to the skin and its number of graphite grains is 3,000 per square millimeter. f

The mechanical characteristics of this material are as follows: I

Tensile strength 90 kilograms per square millimeter=(l28,000 p.s.i.).

Elastic limit 70 kg./sq. mm. (99,56l p.s.i.). Elongation 6%. a

Yield point '-'40-kg./sq, mm. (56,892 p.s.i.).

These characteristics cannot be obtained in cast prodnets of this size (diameter=60 mm.) if they have not received a whitening agent such as magnesium or undergonca thermal treatment comprising the above-described steps such as the hardening from the casting temperature.

I claim:

A high strength malleable i-ron casting having a thickness greater than in. and which is white and free from graphite as cast, containing in addition to iron 2 to 3% carbon, 1 to 2.5% silicon, 0.1 to 2.5% manganese, 0.1 to 3% copper, 0.01 to 0.20% sulphur, and 0.02 to 0.15% phosphorus, a metal of the group consisting of titanium, zirconium and aluminum, in the amount of up to 0.5% in the case of titanium, up to 0.5% in the case of zirconium, and up to 0.20% in the case of aluminum, a whitening metal consisting of at least one member of the group consisting of magnesium, calcium, tellurium, selenium, tin, bismuth, and antimony, the quantity of said whitening metal being the reactive equivalent of 0.03 to 0.20% magnesium, and a promoter metal consisting of at least one member of the group consisting of molybdenum, nickel, copper, manganese, tungsten and vanadium, the quantity of said promoter metal being the reactive equivalent of 0.05 to 0.5 of molybdenum, said reactive equivalent of 1 part by weight of molybdenum being by weight for each of the other promoter metals, 4 parts of nickel, 10' parts of copper, 4 parts of manganese, 2 parts of tungsten and 0.3 partof vanadium, said casting having a structure resulting from treatment by the steps which comprise stripping the-white casting from the mold at a temperature above the eutectoid trans formation temperature which ranges from 750 to 1,000 0, immediately introducing said casting into an oven, stabilizing the temperature of said casting in said oven within the range of 740 to 850 0., effecting after said stabilizing martensitic hardening by immersion in a liquid medium at a temperature up to 250 C., nucleation tempering at a predetermined temperature Within the range 425 to 500 C. for a period of time from 1 to 48 hours, and graphitizing annealing at a temperature within the range 850 to 1,100 C. for a period of time from 15 minutes to 8 hours, said casting containing after said treatment a very large number of very fine, uniformly-distributed, rounded graphite modules.

V llteferenees Cited in the file of this patent UNITED STATES PATENTS 2,578,794 Gagnebin et 1 Dec. 18, 1951 2,595,567 Crome May 6, 1952 2,895,860 Peras July 21, 1959 2,899,346 Peras Aug. 11, 1959 l FOREIGN PATENTS 513,106 Great Britain Oct. 4, 1939 601,755 Great Britain May 12, 1948 768,207 Great Britain 1...; Feb. 13, 1957 

